The cell wall of wood fiber consists of several layers composed primarily of cellulose, hemicellulose and lignin. Pulping is a procedure that disintegrates these fibers by mechanical and chemical means. Ultimately this pulp will be used for papermaking. The objective of pulping is to separate the cellulose fibers and remove as much lignin and hemicellulose as required by the end use but to preserve the fiber strength.
Kraft pulping is a specific pulping process that uses a wide variety of wood sources to produce quality pulps for the manufacture of particularly strong paper. Kraft pulping is a highly alkaline method of wood chip digestion characterized by the use of sodium hydroxide and sodium sulfide in the delignification phase. The kraft process is now the most widely used of all pulping processes. Kraft pulp is commonly used for the production of grocery bag stock and linerboard for corrugated containers and is also incorporated with other pulps into numerous grades of paper, particularly book papers, newsprint, high grade magazine paper, other printing papers, bond and writing papers.
Colored compounds ("chromophores") are created during the kraft cooking process. Chromophores are believed to be derived from either the lignin or hemicellulose components of the fiber, but the chemistry and origin of chromophores are not well characterized. The bulk of chromophores diffuse out of the pulp and are removed in the spent black liquors or in the washing stages of the kraft pulping. However, some chromophores remain in the pulp where they are either physically trapped within the fibers by precipitated xylan or are chemically bound to the hemicellulosic and cellulosic moieties.
Therefore, kraft pulp has a brown color which must be removed before the pulp can be incorporated into writing papers or printing stock. The bleaching process commonly uses elemental chlorine, chlorine dioxide, or hypochlorous acid in aqueous solution. Unfortunately, chlorination results in the formation of large quantities of chlorinated aromatic degradation products that are toxic and difficult to remove by conventional waste treatment. Moreover, the bleaching process damages the pulp fibers by reducing the degree of polymerization of the cellulose, and residual acid left in the paper causes slow degradation over time. Newer bleaching processes employ hydrogen peroxide or, less commonly, oxygen/alkali (O.sub.2 /NaOH), but the bleaching activity of these chemicals is not great enough, particularly for high-yield pulps.
Enzymes are now commonly used to aid the pulping process. In recent years, there have been several reports that xylanases, enzymes that convert xylan to constituent sugars, will improve fiber flexibility in chemical pulps (Mora, et al. 1986, J. Wood Chem. Technol., 6:147-165; Noe, et al. 1986, J. Wood Chem. Technol., 6:167-184). Xylanases are known to remove residual xylan in the production of dissolving pulps (Paice, et al. 1984, J. Wood Chem. Technol., 4:187-198), and to enhance the bleaching of kraft pulps (Viikari, et al. pages 67-69, in: Biotechnology in the Pulp and Paper Industry, Stockholm, 1986; Viikari, et al., pages 151-154, in: Fourth International Symposium on Wood and Pulping Chemistry, Paris, 1987; Chauvet, et al., pages 325-327, in: Fourth International Symposium on Wood and Pulping Chemistry, Paris, 1987). Chauvet, et al., supra 1987, reported that treating kraft pulps with xylanases lowered the lignin content of the pulps and lowered the amount of chemicals necessary for achieving a given degree of brightness. The mechanical strength properties of the sheets were not severely affected. Viikari, et al., supra 1987, has reported reductions of 25-50% in the amount of chlorine dioxide required for bleaching following treatment of kraft pulps with specific xylanases.
Many different microbial xylanase preparations have been reported in the literature. Most are derived from various fungal sources; a few are from Streptomyces and yeasts. See published PCT applications WO 91/02840, WO 91/02839, U.S. Pat. No. 4,966,850 and published European application 0 383 999.
In general, the xylanases that are most useful for facilitating the bleaching of kraft pulps are preparations that are free of cellulase activity. Enzymes that are active at neutral or alkaline pH would also be useful because large pH shifts would not be necessary in order to change the pulp from its alkaline state following the cook. Successive enzyme and alkali extraction steps would then be feasible. Temperature stability is also desirable because higher temperatures speed up enzyme activity and facilitate diffusion of the chromophores out of the pulp. The enzymes should be resistant to inhibition by the kraft degradation products. Enzymes that are capable of penetrating the micropore structure of the fibers would be very useful, because they would have access to a larger fraction of the chromophores. An enzyme that will cleave specifically at or near xylan moieties that are crosslinked into chromophores would be particularly useful, because color removal would be maximized while minimizing yield loss. Other than the present invention, there are no xylanases known to remove chromophores from wood pulp.